Cardiovascular disease, together with other diseases of lipid metabolism, remains the number one cause of health problems and mortality. Although much is known about the metabolism and transport of lipoproteins and lipids by cells from the biochemical and cell biology standpoint, the molecular details and mechanisms by which these physiological processes take place remain poorly understood. A detailed structural description of the plasma lipoproteins, their constituent apolipoproteins, and their receptors is crucial to understanding the molecular mechanisms involved in the physiology of lipoprotein metabolism and the pathophysiology of atherosclerosis and other diseases of lipid metabolism. The long-term objectives of this research are to determine this molecular detail and to understand the structure-function relationships underlying these processes. ApoA-I, the major protein of High Density Lipoprotein (HDL), plays important roles in reverse cholesterol transport. Interaction with the ABCA1 transporter removes cholesterol the cell membrane forming the nascent HDL particle. Activation of LCAT by ApoA-I results in cholesterol esterification, and the transformation to the mature HDL particle. Finally, apoA-I is a ligand for the hepatic SR-B1 receptor through which cholesterol is delivered to the liver. In Specific Aim 1, we will build on or recent advance in understanding of the molecular structure of D(185-243)apoA-I to provide a mechanistic understanding of the molecular features of apoA-1 crucial to understanding the mechanisms of lipid interaction, LCAT binding and activation and HDL formation and function at a molecular level. These studies will use mutations designed through our new structural understanding of apoA-I. In Specific Aim 2, we will correlate the molecular details derived in Aim 1 with functional studies to understand the molecular mechanisms of HDL formation, LCAT interaction, and cholesterol efflux mediated by ABCA1 and ABCG1. Thus, the molecular hypotheses formulated and studied from the structural standpoint in Specific Aim 1, will be probed at the functional level in Specific Aim 2. Our structural studies will provide critical knowledge on these processes. The outcome on completion of these aims will be a significant enhancement in our understanding of the molecular details and interactions of the crucial players in reverse cholesterol transport and metabolism that will drive our ability to develop molecularly based strategies to prevent or control dyslipoproteinemias and diseases of lipid metabolism.